The invention relates to a method of monitoring a photolithographic process whereby a test pattern is imaged a number of times side by side with the same radiant energy but in a series of different irradiation times on a photoresist layer provided on a surface of a substrate, whereupon the photoresist is developed.
The test pattern may be a rectangle. A number of these rectangles is then imaged on the photoresist side by side, said rectangles having a length and a width, for example, of a few millimeters. It is then simple to ascertain with the naked eye at which irradiation time the photoresist has just become soluble in developer during the development of the photoresist. Since the irradiation energy used for this is known, the irradiation dose is thus determined at which the photoresist can be just removed from the surface. This irradiation dose is sometimes called the energy-to-clear.
In the photolithographic process to be monitored, a layer of photoresist is irradiated in a pattern and subsequently developed, such that photoresist patterns are formed. The method mentioned in the opening paragraph may be used for optimizing the process to be monitored, for example as regards the thickness of the photoresist layer, and also to check from time to time whether the process still functions satisfactorily. If such a series of test patterns is imaged in different locations on the substrate, it can also be tested whether the process takes place homogeneously over the surface of the substrate.
It is known to determine the energy-to-clear by means of a rectangular test pattern which is formed by means of a diaphragm. The photoresist is irradiated within the rectangle of the test pattern, but not outside it. The series of irradiation times is chosen so as to lie around a nominal value which corresponds to the expected energy-to-clear. The irradiation times, and thus the irradiation doses in the series, differ from one another, for example, by 2% of the nominal value each time.
It is found in practice that the known method is not suitable for monitoring a photolithographic process in which pulsed laser radiation from, for example, a KrF excimer laser is used for the pattern irradiation. This laser radiation is used in projection devices with which patterns having minimum dimensions of 0.25 .mu.m are imaged on photoresist, for example, in the PAS-5500/90 and the PAS-5500/300 of the ASML Company. An irradiation dose to be realized can be set on the device. When such devices are used, it is found that the accuracy with which a set irradiation dose is realized is practically independent of the value of this dose. It is found in practice that irradiation doses with values of up to approximately 100 mJ/cm.sup.2 can all be realized with the same accuracy of approximately .+-.0.15 mJ/cm.sup.2. Usual pattern irradiations are carried out with irradiation doses of 5 to 20 mJ/cm.sup.2, the test patterns mentioned above accordingly with differences between doses of 0.1 to 0.4 mJ/cm.sup.2. The pattern irradiations can be carried out with a sufficient accuracy, the energy-to-clear irradiations cannot.
The dose necessary for rendering the photoresist just capable of development in addition depends on the size of the pattern imaged on the photoresist. Small patterns must be irradiated longer than large ones. When monitoring the lithography process, where test patterns are preferably used which are observable to the naked eye, this should be taken into account. To render a test pattern imaged in the form of a rectangle having sides of, for example, 2 and 4 mm on the photoresist capable of development, an irradiation dose is necessary which is only half as great as that which is necessary for rendering sub-micron patterns imaged on the photoresist capable of development. Dose differences of as little as 0.05 to 0.2 mJ/cm.sup.2 would accordingly have to be realized for determining the energy-to-clear by means of such a large test pattern.